1. Field of the Invention
The present invention relates to a luminance/color-difference signal separator circuit for separating the luminance signal and the color difference signal which form the composite color picture signal representing an image.
2. Description of the Background Art
Separating the luminance signal, or Y signal, and the color difference signal, or C signal, from the composite color picture signal is a technology vital for reproducing video. There are several types of conventional luminance/color-difference signal separator circuits, or Y/C signal separator circuits. One of the types of separator circuits uses frequency separating filters for separating signals based on the frequency characteristics of the Y signal and the C signal. Another type uses comb filters for separating signals based on the frequency spectra of the signals on the horizontal scanning lines constituting a frame of image. A still another type is an adaptive Y/C signal separator circuit that detects imagewise motions or correlation levels between lines of a picture and adaptively switches filters to be used. Among those circuits, the adaptive Y/C signal separator is best in separator performance.
However, the conventional adaptive Y/C signal separator has the problems described below. A typical adaptive Y/C signal separator is adapted to determine whether or not there is a correlation between horizontal lines and, depending upon the result, switches the filter between the frequency separator filter and the comb filter. Alternatively, the adaptive Y/C signal separator is adapted to select the filter between the two-line comb filter and the three-line comb filter, depending upon a resultant correlation between horizontal lines. Further alternatively, the adaptive Y/C signal separator is adapted to determine whether or not there is a motion in the picture and, depending upon the result, switches the filter between the line comb filter and the frame comb filter. The thus selected filter causes the color difference, or chrominance, signal to be extracted. Then, the adaptive Y/C signal separator subtracts the extracted C signal from the composite signal to extract the Y signal.
However, the correlation or the motion of an actual image differs from line to line. Therefore, the adaptive Y/C signal separator including the filters adapted to be switched depending upon the results determined under the same conditions would sometimes cause an abrupt change in the filtering characteristics to prevent the image from being separated correctly. In addition, because the filters are switched from one filter to another, the filtering at the transition point of switching is not always best. This means that, even though the filter selection is adaptive, the separated Y and C signals may interfere each other so that the reproduced image is sometimes deteriorated.
It is an object of the present invention to provide a luminance/color-difference signal separator circuit with the problems described above overcome.
It is a more specific object of the invention to provide a luminance/color-difference signal separator circuit, in which the deterioration of an image that would otherwise be caused by adaptively switching filters is minimized.
In accordance with the present invention, the luminance/color-difference signal separator circuit comprises a motion detector for detecting a motion of the image and determining a motion level; a line correlation circuit for detecting a correlation level of lines of the image; a first filter for extracting the color difference signal from the composite color picture signal according to first filtering characteristics; a second filter for extracting the color difference signal from the composite signal according to second filtering characteristics different from the first characteristics; a first weighting circuit for calculating, based on the motion level, a weighted average of the color difference signal output from the first and second filters; a third filter for extracting the color difference signal from the composite signal according to third characteristics different from the first and second characteristics; a second weighting circuit for calculating, based on the correlation level, the weighted average of the color difference signal output from the second and third filters; a selector for selecting and outputting an output signal of the first weighting circuit as an ultimate color difference signal if the motion detector determines that the motion of the image is below a predetermined threshold, and, if the motion of the image exceeds the predetermined threshold, selects and outputs the output signal from the second weighting circuit as the ultimate color difference signal; and a subtracter for subtracting the ultimate color difference signal from the composite signal to produce an ultimate luminance signal, the color difference signal being selected by the selector.
This configuration allows the first filter, the second filter and the third filter to extract the color difference signal according to their own characteristics. The first weighting circuit calculates the weighted average of the color difference signal output by the first and second filters according to the motion level of the image. Therefore, the signal output from the first weighting circuit represents a signal generated by mixing the signals from the first and second filters according to the ratio or weight responsive to the motion level. Therefore, even when the motion level varies, the change in the filter characteristics affects the final color difference signal smoothly. The second weighting circuit calculates the weighted average of the color difference signal output by the second and third filters according to the correlation level of the image. Therefore, the signal output from the second weighting circuit represents a signal generated by mixing the signals from the second and third filters according to the ratio or weight responsive to the correlation level. Therefore, even the correlation level varying allows the change in the filter characteristics to less affect the final color difference signal.
If the motion of the image is below the predetermined threshold, the selector then selects the output signal from the first weighting circuit. If the motion exceeds the predetermined threshold, then the selector selects the output signal from the second weighting circuit. The signal selected by, and output from, the selector is the final color difference signal. The subtracter subtracts the obtained color difference signal from the composite signal to produce the final luminance signal. The characteristics of the second filter is adaptively reflected both on the output signal from the first weighting circuit and on the output signal from the second weighting circuit. Therefore, even when the color difference signal selected by the selector is switched, the switching is performed much smoother than when the color difference signal extracted by the first filter is switched to the color difference signal extracted by the third filter.